Abstract

Aspirated systems such as those used for smoke detection are characterised by the use of long (e.g. lOOw), small- ore (e.g. 2lmm) pipes with sampling holes (e.g. 2mm) drilled at regular intervals (e.g. 4m). A low-power aspirator (e.g. 2W) draws a continuous sample of air from each hole throughout the monitored zone, via each pipe to a highly sensitive smoke detector. Critical objectives in the design of such systems are to achieve the highest possible "balance" (the distribution of hole flow rates and hence the effective smoke-sensitivity throughout the zone), commensurate with the lowest possible smoke "transport time" (from a hole to the detector). These two objectives are often in conflict. Air flowing into the sampling hole constitutes an induction jet that disturbs the upstream flow regime. This disturbance, or that of a pipe bend, causes the established pipe velocity profile to reset to plug flow. The core velocity growth profile of the initially-disturbed, developing flow regime has been determined experimentally for a range of Reynolds numbers, having an effectivity length of up to 500 diameters. The range found relevant to aspirated pipe systems embraces the laminar, transitional and turbulent flow regions (400 < Re < 4000). This family of growth profiles is used to determine the transport time for smoke entering a given hole, using a Time Factor algorithm. Dilution of the smoke due to mixing and due to the induction of fresh air from other holes is taken into account. The disturbance also causes a local increase in the friction factor that affects the downstream pressure drop for at least 100 diameters. The friction factor for a range of disturbance levels and Reynolds numbers has been determined experimentally. The pressure drop in the vicinity of the hole is further increased as a result of the force required to accelerate the induction jet. Moreover, the hole flow rate is determined by the local pressure differential, the size of the hole and the size of the pipe, but this flow is enhanced by the upstream flow rate, in a phenomenon described as Ultrafiow. External air flows typically caused by building ventilation and described as crosswind, cause a reduction in the hole flow rate in a phenomenon described as Infraflow. By superposition, these phenomena determine the net flow rate of each hole and therefore the flow rate in each pipe segment (between holes). This, coupled with the local friction factor determines the pressure distribution throughout the system. Since the local pressure differential determines the hole flow, all elements of the system are interactive and a computer program is required to obtain the system operating point by rapid iteration.